Fiberboard compartment-forming inserts for a rigid shelving unit are an advantageous construction for office use and the like. If the fiberboard inserts are collapsible and formed so as to be interchangeable between those forming wide horizontal compartments, those forming tall vertical compartments, and combinations thereof, a single rigid shelf unit can be converted easily to current demands. The fiberboard portions of such units should be easy to assemble, without conventional nuts and bolts, yet be sufficiently sturdy to hold the weight of the material stored therein and the wear of sorting usage, for instance when such a unit is used once or twice daily for sorting office mail or for holding periodicals in a library or the like.
Means have been sought to provide an easy to assemble fiberboard insert that nonetheless, once assembled, cannot be dislodged from the shelf on which it sits. For instance, the inserts disclosed in U.S. Pat. No. 4,062,302 are provided with partitions that fold outwardly so as to be trapped behind the side posts of the rigid shelving units. When an open-framework rigid shelving unit is utilized, however, it is desirable to provide rear and side fiberboard walls and to have the insert, including shelve edges and side walls, flush with the open end of the rigid shelving unit, so that material stored therein cannot be trapped behind the side posts. To achieve this end, the side walls, providing support to at least horizontally disposed fiberboard partitions, should extend up to the open end. Such a design precludes trapping portions of the fiberboard insert behind the side posts.